Sweet and umami (the taste of glutamate) tastes play a major role on the perception of calorically rich and essential nutrients. In humans, three members of the T1R class of taste-specific G protein-coupled receptors (T1R1, T1R2, and T1R3), which reside on chromosome 1, are known to function in combination as heterodimeric receptors for sweet and umami tastes. The T1R proteins each have a large amino-terminal extracellular domain, which is thought to be the site of ligand binding. Unlike the T2R (bitter) taste receptors, T1R proteins are encoded by large genes that span 3 kb to 23 kb in the genome; T1R1 and T1R2 both have six exons, while T1R3 has seven exons.
It is proposed that single nucleotide polymorphisms (SNPs) or variant haplotypes of the T1R genes in humans may underlie individual differences in the perception and recognition threshold for sweeteners and amino acids.